Method for forming a metal article

ABSTRACT

A method for forming a metal article includes: heating a mold to a working temperature ranging from 350° C. to 550° C., the mold having a forming surface with a predetermined pattern; disposing a metal sheet in the mold that has been heated, the metal sheet being made from one of aluminum alloy and magnesium alloy and having a temperature not higher than 100° C.; and forming the metal sheet by applying a fluid-pressure of not less than 10 kg/cm 2  into the mold to press the metal sheet against the predetermined pattern.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of Taiwanese application no. 099105630, filed on Feb. 26, 2010.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for forming a metal article.

2. Description of the Related Art

Conventionally, a metal article is formed by die casting a molten metal material, or by press forming a metal sheet. By virtue of the die casting method, a metal article may have a relatively complicated structure. However, the casted metal article is likely to deform if it has a thickness of less than 1.0 mm. Furthermore, the casted metal article has a rough surface, which can be modified only by painting or coating techniques, and has no metallic appearance, and thus, is not appealing to consumers.

Moreover, the press forming method can be used for forming a metal sheet having a predetermined thickness into a metal article with desired size and thickness, and is normally conducted at a room temperature or a relatively low temperature. However, such method is hard for forming the metal sheet into a metal article having a complicated structure. Since the metal sheet is originally able to provide smooth, flat, and fine surfaces, the obtained metal article is able to be further modified by a surface treatment such as anodic treatment, electroplating or physical vapor deposition to achieve a required metallic and appealing appearance.

In recent years, an air-pressure forming method was developed, which is mainly for forming a fine grain metal material having a grain size of about 10 micron, such as magnesium alloy AZ31B and aluminum alloy 5083. Generally, the finer the grain size, the better the plasticity of the metal material. Especially, the magnesium alloy AZ31B is commonly used for making a housing of a notebook computer, a mobile phone, etc. through the air-pressure forming technique.

In the conventional air-pressure forming of a metal sheet made of a magnesium alloy, the metal sheet is preheated for at least 10 minutes before disposing into a forming mold. In order to prevent the plasticity of the metal sheet from being adversely affected, the preheating is conducted to provide the metal sheet having a temperature not lower than the working temperature when the metal sheet is disposed into the forming mold. Thus, the preheating temperature is the same as or slightly higher than a working temperature in the forming mold. Commonly, the thermoplastic metal sheet is known to have better plasticity if the working temperature in the forming mold is relatively high. Although the working temperature for the magnesium alloy has a theoretical upper limit of 550° C., the working temperature for the magnesium alloy is lower than 450° C. in practice. This is because the magnesium alloy may be oxidized when it is preheated at the temperature over 450° C. for a relatively long period of time, and the subsequent surface treatment will be adversely affected. Accordingly, the working temperature for the magnesium alloy is set to about 370° C.

Referring to FIG. 1, an air-pressure formed housing 9 was made by forming a magnesium alloy metal sheet of 0.6 mm thickness at the working temperature of 370° C. for 240 seconds. The housing 9 has a maximum thickness (0.58 mm) at a middle portion of a base wall 91, and a minimum thickness (0.52 mm) at two curve portions 93 connecting the base wall 91 and two side walls 92. The maximum value of thickness variance of the metal sheet is 15% [i.e., (0.6÷0.52−1)×100%], and a thickness ratio (maximum thickness/minimum thickness) of the housing 9 is 1.12 (i.e., 0.58÷0.52=1.12). In this case, a radius of curvature of the curve portion 93 (i.e., R value) should be larger than 3 mm. The smaller the R value means that the structure of the metal article is more complex. The housing 9 having the R value of larger than 3 mm, in practice, may also be formed using the conventional press forming method. Since the forming performance and structural complexity of the metal article made using the air-pressure forming method can be achieved by the conventional press forming method, it is not easy for the air-pressure forming method to be popularized in the field of metal forming.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a method for forming a metal article that can overcome the aforesaid drawbacks associated with the prior art.

Accordingly, a method for forming a metal article of this invention comprises:

heating a mold to a working temperature ranging from 350° C. to 550° C., the mold having a forming surface with a predetermined pattern;

disposing a metal sheet in the mold that has been heated, the metal sheet being made from one of aluminum alloy and magnesium alloy and having a temperature not higher than 100° C.; and

forming the metal sheet by applying a fluid-pressure of not less than 10 kg/cm² into the mold to press the metal sheet against the predetermined pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiments of the invention, with reference to the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of a housing made using a conventional air-pressure forming method;

FIG. 2 is a flow chart showing the preferred embodiment of a method for forming a metal article according to the present invention;

FIG. 3 is an exploded perspective view of a mold used for forming the metal article according to the present invention;

FIGS. 4 to 6 show successive steps of the method for forming the metal article according to the present invention;

FIG. 7 is a perspective view illustrating the first preferred embodiment of a metal article according to the present invention;

FIG. 8 is a cross-sectional view of FIG. 7;

FIG. 9 is a perspective view illustrating the second preferred embodiment of a metal article according to the present invention;

FIG. 10 is a cross-sectional view of FIG. 9;

FIG. 11 is a perspective view illustrating the third preferred embodiment of a metal article according to the present invention;

FIG. 12 is a perspective view illustrating a metal article used for evaluating examples and comparative examples; and

FIGS. 13 to 20 are electron microphotographs to illustrate microstructures of test samples 1 to 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before the present invention is described in greater detail with reference to the accompanying preferred embodiments, it should be noted herein that like elements are denoted by the same reference numerals throughout the disclosure.

FIGS. 2 to 6 illustrate the preferred embodiment of a method for forming a metal article according to the present invention.

In step 101, a mold 6 that has a forming surface 60 with a predetermined pattern 61 is prepared using an engraving method or a sandblasting method. The engraving method may be conducted using a laser beam, an etchant, or a graver. For details of the mold 6 other than the predetermined pattern 61, one may refer to the contents disclosed by the inventor of this application in Taiwan patent publication no. 201008675, the disclosure of the Taiwanese patent publication is incorporated herein by reference in its entirety.

The mold 6 can be modified based on the desired shape and pattern of the metal article, and should not be limited to this embodiment.

Preferably, the predetermined pattern 61 is formed using the sandblasting method, by which the predetermined pattern 61 in the mold 6 can be effectively transferred to a metal sheet 5 (see FIGS. 3 to 6). The metal sheet 5 is made from one of aluminum alloy and magnesium alloy. Examples of the aluminum alloy include aluminum alloys 6061, 5052, 5182, 5083, etc., and examples of magnesium alloy include magnesium alloys AZ31B, AZ61, etc. In the preferred embodiment, the predetermined pattern 61 is formed by the following steps:

(a) disposing a mask (not shown) on the forming surface 60 of the mold 6, the mask exposing a portion of the forming surface 60 corresponding to the predetermined pattern 61;

(b) sandblasting the portion of the forming surface 60 exposed from the mask; and

(c) removing the mask from the forming surface 60 so that the forming surface 60 has a gloss portion 62 not treated by sandblasting and a matte portion 63 that has been treated by sandblasting.

In this case, the gloss portion 62 is smooth, and the matte portion 63 is rough.

More preferably, the mask is a tape that has a pattern exposing the portion of the forming surface 60 and that is detachably attached to the forming surface 60.

In step 102, the mold 6 is heated to a working temperature ranging from 350° C. to 550° C. by virtue of a heating device 64 of the mold 6. Preferably, the working temperature ranges from 450° C. to 550° C.

In step 103, the metal sheet 5 is disposed in the mold 6 that has been heated. When being disposed into the mold 6, the metal sheet 5 is preheated to have a temperature not higher than 100° C. The metal sheet 5 can also be directly disposed in the mold 6 without being preheated.

In step 104, the metal sheet 5 is formed by applying a fluid-pressure of not less than 10 kg/cm² into the mold 6 to press the metal sheet 5 against the predetermined pattern 61. Thereafter, the metal sheet 5 is formed into the metal article and is bent at least one corner of the metal article. The metal article has a minimum thickness where the corner is formed and a maximum thickness where no corner is formed. A ratio of a maximum thickness to the minimum thickness is not less than 1.5. The fluid may be air or oil. The fluid-pressure is preferably not less than 30 kg/cm².

In this preferred embodiment, the fluid is air. The air is pressurized using a pressurization device 65 to the fluid pressure for introducing into the mold 6. The forming time for the metal sheet 5 ranges from 15 seconds to 2 minutes, which is based on a deformation degree of the metal sheet 5. For example, when the R value is relatively small (i.e., the pattern required to be formed on the metal article is relatively fine), the required forming time is relatively long. When the R value is relatively large (i.e., the deformation degree of the metal sheet 5 is relatively small), the required forming time is relatively short.

Furthermore, the metal article can be removed from the mold 6 by the method disclosed in Taiwan patent publication no. 201008675.

The metal article has an outer surface, an inner surface opposite to the outer surface, and a molded pattern transferred from the predetermined pattern 61 of the mold 6. The molded pattern includes at least one protrusion protruded from the outer surface, and at least one recess corresponding to the protrusion and indented from the inner surface. The molded pattern not only serves as a decorative pattern, but also enhances the rigidity of the metal article.

It is found that when the metal sheet 5 is directly disposed into the mold 6, in which the metal sheet 5 is preheated at a temperature not higher than 100° C. or is not preheated, the molded pattern on the metal article is not as anticipated to have a crack. On the contrary, the metal sheet has an improved plasticity. That is to say, if the metal sheet is preheated for a relatively short time or is not preheated to be directly disposed into and to be formed immediately in the mold 6, the metal sheet 5 has an unexpectedly improved initial plasticity, compared to the metal sheet that is preheated to have a temperature not lower than the working temperature as disclosed in the prior art.

FIGS. 7 and 8 illustrate the first preferred embodiment of the metal article 2 made according to the method of this invention. The metal article 2 is a housing, and includes a quadrilateral base wall 21 and four side walls 22. Each of the four side walls 22 connects to the quadrilateral base wall 21 via a first curved portion 23, and connects to an adjacent one of the side walls 22 via a second curved portion 24. Outer faces of the quadrilateral base wall 21 and the side walls 22 constitute an outer surface 25 of the metal article 2, while inner faces thereof constitute an inner surface 26. The metal article 2 is formed with a molded pattern 27. In the first preferred embodiment, the molded pattern 27 includes a plurality of protrusions 271 protruded from the outer surface 25, and a plurality of recesses 272. Each of the recesses 272 is indented from the inner surface 26 and is formed corresponding to one of the protrusions 271. The molded pattern 27 can be varied based on design options, and may be formed on the first and second curved portions 23, 24. That is to say, the protrusions 271 should not be limited to a strip form, and the pattern 27 can be formed on a part or the whole of the metal article 2.

The molded pattern 27 is transferred from the predetermined pattern 61 on the forming surface 60 of the mold 6 during the step 104. Accordingly, the outer surface 25 of the metal article 2 has a matte portion (i.e., the protrusions 271), and a gloss portion (i.e., the outer surface 25 excluding the protrusions 271). Since it is easy to perceive a contrast between the matte and gloss portions on the outer surface 25, the metal article 2 may be made to have diversification of pattern designs.

Besides, since the metal sheet 5 has an unexpectedly improved initial plasticity when being processed using the method of this invention, the predetermined pattern 61 of the mold 6, which includes a recess groove having a groove width ranging from 0.05 mm to 1 mm (preferably from 0.05 mm to 0.1 mm), may also be transferred to the metal article 2. Therefore, the molded pattern 27 may be finer than that of the prior art. When the groove width of the recess groove of the predetermined pattern 61 is 0.05 mm, the molded pattern 27 may have a height of 0.01 mm that is transferred from the recess groove of the predetermined pattern 61.

FIGS. 9 and 10 illustrate the second preferred embodiment of the metal article 3 made according to the method of this invention. The second embodiment differs from the first embodiment only in that the metal article 3 is adapted to be a monitor cover of a notebook computer, and includes an arc wall 31 and the molded pattern 27. The arc wall 31 has outer and inner surfaces 311, 312. The protrusions 271 protrude from the outer surface 311 of the arc wall 31, while the recesses 272 are indented from the inner surface 312 of the arc wall 31.

FIG. 11 illustrates the third preferred embodiment of the metal article 7 made according to the method of this invention. The third embodiment differs from the first embodiment only in that the metal article 7 is in a shape of a bucket, and includes a base wall 71, a surrounding wall 72 extending upwardly from the base wall 71, and the molded pattern 27. The surrounding wall 72 has outer and inner surfaces 721, 722. The protrusions 271 protrude from the outer surface 721 of the surrounding wall 72, while the recesses 272 are indented from the inner surface 722 of the surrounding wall 72. The metal article 7 may be formed from a bucket-shaped performed body. Besides, the metal article 7 may be formed in a shape of a tube by removing the base wall 71.

The present invention is explained in more detail below by way of the following examples.

EXAMPLES Examples 1˜5 (Ex 1˜5) and Comparative Examples 1˜7 (CE 1˜7)

Metal articles of Examples 1˜5 and Comparative Examples 1˜7 were formed using metal sheets each being made of magnesium alloy AZ31B and having a thickness of 0.6 mm. The mold (not shown) used for forming the metal articles has a mold cavity (size: 76 mm×42 mm×5 mm). Each metal article for Examples 1˜5 and Comparative Examples 1˜7 is formed as shown in FIG. 12, and is noted as “4”. In Examples 1˜5 and Comparative Examples 1˜7, each metal article 4 has an R value of 2 mm between two adjacent side walls 42, 43, and an R value of 0.5 mm between a base wall 41 and one of the side walls 42, 43. FIG. 12 is only for illustration, and the size of the metal article 4 shown therein may be not in proportion to that of the actual metal article.

The preheating time, the preheating temperature, the working temperature, the fluid-pressure, and the forming time for each of Examples 1˜5 and Comparative Examples 1˜7 are shown in Table 1. In Examples 1, 2, 4 and 5, each metal sheet was disposed directly into the mold without being pre-heated. After the mold is closed, air was introduced into the mold and pressurized to provide the fluid-pressure to form each metal sheet for 2 minutes. Thereafter, each metal article 4 was obtained.

In Example 2, the working temperature in the mold had an initial temperature of 420° C., and was raised to 480° C. with the introduction of the air. As for the other examples or comparative examples, the mold was controlled to have a constant temperature as shown in Table 1.

In Example 3 and Comparative Examples 1˜7, the metal articles 4 were prepared following the procedure employed in Example 1 except that the metal sheets were preheated before being disposed into the mold.

In Examples 1˜5 and Comparative Examples 1˜7, the thicknesses and appearances of the metal articles 4, and the maximum values of the thickness variance for the metal sheets are listed in Table 2. Referring to Table 2 and FIG. 12, “A” means a mean value of the thicknesses of corners A1, A2, A3, and A4 for each metal article 4; “B” means a mean value of the thicknesses of corners B1 and B2; “C” means a mean value of the thicknesses of corners C1 and C2; and “D” means the thickness of a central portion of the base wall 41. The thickness of the corner B1 was sampled at a middle point between the corners A1 and A2. The corners B2, C1, and C2 were also sampled by the same way as that of the corner B1.

The maximum value of thickness variance of the metal sheet is equal to [(an original thickness of the metal sheet÷A−1)×100%]. The original thickness of the metal sheet is 0.6 mm.

TABLE 1 Preheating Preheating Working Fluid- Forming temp. time temp. pressure time (° C.) (minutes) (° C.) (kg/cm²) (minutes) Ex 1 — — 480 40 2 Ex 2 — — 420~480 40 2 Ex 3 100 5 480 40 2 Ex 4 — — 480 30 2 Ex 5 — — 370 30 2 CE 1 480 1 480 40 2 CE 2 480 3 480 30 2 CE 3 480 5 480 30 2 CE 4 370 3 370 30 2 CE 5 370 5 370 30 2 CE 6 370 10  370 30 2 CE 7 200 5 480 40 2

TABLE 2 Thickness Thickness Vari- (mm) ratio ance* Appear- A B C D D/A D/B D/C (%) ances Ex 1 0.118 0.331 0.341 0.543 4.60 1.64 1.59 408 ⊚ Ex 2 0.121 0.330 0.342 0.542 4.48 1.64 1.58 396 ⊚ Ex 3 0.122 0.338 0.349 0.544 4.46 1.61 1.56 392 ⊚ Ex 4 0.132 0.347 0.358 0.548 4.15 1.58 1.53 355 ⊚ Ex 5 0.363 0.421 0.425 0.554 1.53 1.32 1.30 65 ⊚ CE1 0.127 0.366 0.370 0.546 — — — 372 X CE2 0.173 0.376 0.379 0.551 — — — 247 XX CE3 0.219 0.392 0.404 0.549 — — — 174 XX CE4 0.380 0.429 0.430 0.553 1.46 1.29 1.29 58 Δ CE5 0.383 0.443 0.447 0.557 1.45 1.26 1.25 57 Δ CE6 0.402 0.454 0.461 0.561 1.40 1.24 1.22 49 Δ CE7 0.142 0.349 0.355 0.547 3.85 1.57 1.54 323 ⊚ *The variance means “the maximum value of thickness variance of a metal sheet.” *A thickness measurement error in Table 2 is ±0.005 mm. *When the metal article 4 does not has defects, it is represented by “⊚”. *When the metal article 4 has a defective formation, it is represented by “Δ”. *When the metal article 4 has a crack or a hole at corners, it is represented by “X”. *When the metal article 4 has a crack or a hole at corners and an oxidized surface, it is represented by “XX”.

In Examples 1 and 3 and Comparative Example 7, the metal sheets were formed at the same forming temperature under the same fluid-pressure. The metal sheet of Example 1, which was not preheated, has a relatively large thickness variation. With the increase of the preheating temperature, the thickness variation of the metal sheet becomes worse (i.e., the plasticity of the metal sheet will become worse).

In Examples 1 and 4, the metal sheets were formed under the same working temperature of 480° C. The metal sheet in Example 1 was formed under the fluid-pressure of 40 kg/cm² that is higher than the fluid-pressure (30 kg/cm²) used in Example 4, and was formed to have a greater variance in thickness. Accordingly, with the same forming temperature, the plasticity of the metal sheet can be enhanced by the increase of the fluid-pressure.

In Examples 4 and 5, the metal sheets were formed under the same fluid-pressure of 30 kg/cm². The metal sheet in Example 4 was formed under 480° C. to have a maximum value of thickness variance of 355%. The metal sheet in Example 5 was formed under 370° C. to have a maximum value of thickness variance of 65%, which is much lower than that of Example 4. Accordingly, under the same fluid-pressure, the metal sheet formed in 480° C. has better plasticity than the metal sheet formed in 370° C.

In Example 5 and Comparative Examples 4˜6, the metal sheets were formed under the same working temperature of 370° C. The metal sheet in Example 5 was not preheated, and the metal sheets in Comparative Examples 4˜6 were preheated at 370° C. for 3˜10 minutes. It is found that the plasticity of the metal sheet becomes worse (i.e., the thickness variance of the metal sheet becomes smaller) with the increase of the preheating time. Besides, when the working temperature is set to 370° C., the fluid-pressure has an upper limit of 30 kg/cm². If the working temperature is 370° C. and the fluid-pressure is up to 40 kg/cm², an air leakage of the mold will occur. However, in Examples 1˜3, no air leakage occurred even when the fluid-pressure reached 40 kg/cm². This is because that the metal sheet at 480° C. has better plasticity and can be further deformed when a larger fluid-pressure is applied.

From the results of Examples 1 and 2, it is found that when the working temperature of the mold is a constant value or progressively increases, both of the metal articles 4 are well-formed.

In Comparative Examples 1˜3, the metal sheets were preheated at 480° C., and each of the metal articles 4 has a crack or a hole at a corner. With the increase of the preheating time, the surfaces of the metal articles 4 were also oxidized. Besides, the metal sheet in the mold under constant fluid-pressure and working temperature has poor plasticity if the preheating time is longer (see Comparative Examples 2 and 3). This is because the preheated metal sheet has lost a portion of its plasticity. The longer the preheating time, the worse the plasticity of the metal sheet is. Accordingly, a crack or a hole may appear at a corner of the metal article 4.

Furthermore, the metal sheet in Comparative Example 1 was preheated for 1 minute, followed by immediately being disposed into the mold. Thus, the metal sheet in Comparative Example 1 can be formed under the fluid-pressure of 40 kg/cm². However, the metal sheets in Comparative Examples 2 and 3 were preheated for 3 and 5 minutes, respectively, and they cannot be formed under the fluid-pressure of 40 kg/cm². This is because the preheated metal sheets in Comparative Examples 2 and 3 have already lost much of their plasticity.

It is noted that the metal article 4 has a relatively poor impact resistance at a flat portion (i.e., D portion) and a relatively good impact resistance at a corner portion (i.e., A, B, and C portions). As shown in Tables 1 and 2, under the same forming conditions, the thickness ratios of D/A, D/B and D/C of the metal articles 4 of the Examples 1˜5 are greater than those of the Comparative Examples CE4˜CE7. Hence, the metal articles 4 of the Examples 1˜5 can be expected to have improved impact resistance than that of the metal articles of the Comparative Examples CE4˜CE7 in addition to reduction in the weight of the metal article 4.

The maximum value of thickness variance of the metal sheet can be improved by the method of this invention (see Examples 1˜5, especially Examples 1˜4), compared to the maximum value of thickness variance of the metal sheet (15%) described previously in the background part. Besides, the forming time for the metal sheet in this invention can also be reduced. Accordingly, it is anticipated that a finer pattern can be formed on a metal article 4, since the plasticity of the metal sheet can be greatly improved using the method of this invention.

Example 6

A metal article 4 of Example 6 was prepared following the procedure employed in Example 1 except that the mold cavity had a size of 82 mm×45 mm×6 mm, and that the metal article 4 had an R value of 3 mm between two adjacent side walls 42, 43, and an R value of 3 mm between a base wall 41 and one of the side walls 42, 43 (see FIG. 12). In Example 6, the forming time for the metal sheet was 30 seconds, and the properties of the metal article 4 are listed in Table 3.

TABLE 3 Thickness Thickness Vari- (mm) ratio ance* Appear- A B C D D/A D/B D/C (%) ances Ex 6 0.308 0.453 0.459 0.564 1.83 1.25 1.23 95 ⊚ *The variance means “the maximum value of thickness variance of a metal sheet.” *A thickness measurement error in Table 2 is ±0.005 mm. *When the metal article 4 does not have defects, it is represented by “⊚”.

From the results of Example 6, it is found that the metal article 4 can be formed to have a relatively large R value in a relatively short amount of time (30 seconds). Furthermore, the maximum value of thickness variance of the metal sheet in Example 6 can be up to 95%.

Examples 7˜14

Metal sheets of Examples 7˜14 were each formed in a mold at a working temperature shown in Table 4 under a fluid-pressure of 30 kg/cm² for 2 minutes. Before being disposed in the mold, the metal sheets of Examples 7 and 11 were not preheated, and the others were preheated at a preheating temperature as shown in Table 4. Each metal sheet was made of magnesium alloy AZ31B and had a thickness of 0.6 mm. The mold had a pattern on a forming surface thereof. The pattern is a recess groove, which has a groove length of 10 mm, a groove depth of 0.8 mm, and a groove width of 2 mm. The pattern on the mold can be transferred to the metal sheet to form a protrusion protruded from an outer surface of the metal sheet to have a height.

TABLE 4 Preheating Preheating Working temp. time temp. Height Example (° C.) (minutes) (° C.) (mm) 7 — — 480 0.744 8 480 1 480 0.673 9 480 3 480 0.506 10 480 5 480 0.362 11 — — 370 0.203 12 370 1 370 0.188 13 370 3 370 0.136 14 370 5 370 0.097 * “Height” means the height of the protrusion on the metal sheet.

From the results, it is also found that the deformation of the metal sheet to extend into the recess groove in the mold is more difficult, if the metal sheet had been preheated for a relatively long time and if the metal sheet was formed at a relatively low working temperature (370° C.).

<Metallographic Examination>

Eight test examples, each of which was made of magnesium alloy AZ31B, were prepared for the metallographic examination. Test sample 1 was not heated. Test samples 2˜8 were respectively heated at 450° C. for 10 seconds, 20 seconds, 30 seconds, 1 minute, 2 minutes, 3 minutes, and 4 minutes as shown in Table 5, followed by quenching. The results are shown in FIGS. 13 to 20.

TABLE 5 Test samples 1 2 3 4 5 6 7 8 Heating time — 10 seconds 20 seconds 30 seconds 1 minute 2 minutes 3 minutes 4 minutes Photos FIG. 13 FIG. 14 FIG. 15 FIG. 16 FIG. 17 FIG. 18 FIG. 19 FIG. 20

From the results shown in FIGS. 13 to 20, it is known that the grain size of magnesium alloy increased with the increase of the heating time. As shown in FIGS. 13 and 14, after heating at 450° C. for 10 seconds, the magnesium alloy of the teat sample 2 has an increased grain size, compared to that of the test sample 1. After the magnesium alloy was heated for 4 minutes, the grain size of the magnesium alloy in the test sample 8 (see FIG. 20) is greater than that in the test sample 1 (see FIG. 13) by several times, and thus, plasticity is reduced. Accordingly, the preheated metal sheet, which has a relatively large grain size, would have poor plasticity.

While the present invention has been described in connection with what are considered the most practical and preferred embodiments, it is understood that this invention is not limited to the disclosed embodiments but is intended to cover various arrangements included within the spirit and scope of the broadest interpretations and equivalent arrangements. 

1. A method for forming a metal article, comprising: heating a mold to a working temperature ranging from 350° C. to 550° C., the mold having a forming surface with a predetermined pattern; disposing a metal sheet in the mold that has been heated, the metal sheet being made from one of aluminum alloy and magnesium alloy and having a temperature not higher than 100° C.; and forming the metal sheet by applying a fluid-pressure of not less than 10 kg/cm² into the mold to press the metal sheet against the predetermined pattern.
 2. The method of claim 1, wherein the working temperature ranges from 450° C. to 550° C.
 3. The method of claim 1, wherein the metal sheet is not preheated when disposed in the mold that has been heated.
 4. The method of claim 1, wherein the fluid-pressure is not less than 30 kg/cm².
 5. The method of claim 1, wherein the predetermined pattern is formed by the following steps: (a) disposing a mask on the forming surface of the mold, the mask exposing a portion of the forming surface corresponding to the predetermined pattern; (b) sandblasting the portion of the forming surface exposed from the mask; and (c) removing the mask from the forming surface so that the forming surface has a gloss portion not treated by sandblasting and a matte portion that has been treated by sandblasting.
 6. The method of claim 1, wherein, during the step of forming the metal sheet, the metal sheet is formed into the metal article and is bent at least one corner of the metal article, the metal article having a minimum thickness where the corner is formed, and a maximum thickness where no corner is formed, a ratio of the maximum thickness to the minimum thickness being not less than 1.5. 